1. Field of the Invention
The present invention pertains generally to a light source device including a dielectric barrier discharge fluorescent lamp that fires utilizing ultraviolet light generated by dielectric barrier discharge. More specifically, the invention is directed to a light source device for use in image readout devices capable of light emission in which a dielectric barrier discharge fluorescent lamp is synchronized with an external synchronization signal without attendant fluctuation in optical power. A phase comparator compares the oscillation signal phase of a variable frequency oscillator divided by a frequency divider with an external synchronization signal. The phase comparator controls the oscillation frequency of a variable frequency oscillator as a function of the phase difference. Consequently, the oscillation phase of the variable frequency oscillator is phase locked by the external synchronization signal. The oscillation signal of the variable frequency oscillator is input to the gate signal generation circuit, and the switch devices of an inverter circuit are opened and closed by the output of a gate signal generation circuit. The direct current voltage output by a DC power source is converted into alternating current voltage and is output by an inverter circuit and is applied to a lamp through a boosting transformer in order to light the lamp.
2. Description of the Related Art
Light source devices including dielectric barrier discharge fluorescent lamps are conventionally used as light sources for image readout devices. FIG. 10 is a diagram showing a constituent example of the lighting circuit of a dielectric barrier discharge fluorescent lamp 1 utilizing a push-pull inverter circuit while FIG. 11 is a diagram showing the operation of the inverter circuit presented in FIG. 10. Dielectric barrier discharge fluorescent lamp 1 includes a boosting transformer 2, and an inverter circuit 3 comprising switch devices Q1, Q2 connected on the primary winding of boosting transformer 2. Alternating current voltage is applied to the primary side of boosting transformer 2 by alternately turning on switch devices Q1, Q2, and lamp 1 is lit.
The sawtooth wave shown in FIG. 11(a) is output by sawtooth wave oscillator 11 and is input to comparator Cmp which compares the sawtooth wave with the voltage Vs at a fixed level, and generates output when the sawtooth wave exceeds a fixed level signal Vs. For this reason, a pulse signal having a prescribed cycle is output as shown in FIG. 11(b) from comparator Cmp when a sawtooth wave is input. This pulse signal is input to clock terminal CLK of flipflop FF of gate signal generation circuit 4 and flipflop FF is inverted as shown in FIG. 11(c) by this pulse signal. The output Q of flipflop FF and its inverted output Qxe2x80x2 (a horizontal line is appended over Q in the diagram, the same hereinafter) are input to the input terminals of gate circuits G1, G2, and the pulse signal output by the comparator Cmp is input to the other input terminal of aforementioned gate circuits G1, G2.
Accordingly, a 2-phase pulse signal is output from gate circuits G1, G2, as shown in FIGS. 11(d) and 11(e), and switch devices Q3, Q4 are alternately turned on by the 2-phase pulse signal. The output of switch devices Q3, Q4 is applied to the gate terminals of the switch devices Q1, Q2 via resistors R1, R2 as gate signals GU, GL for inverter circuits. In so doing, switch devices Q1, Q2 are alternately turned on, voltage is applied to lamp 1 as shown in FIG. 11(f) to light the lamp 1. Since light emission of the dielectric barrier discharge fluorescent lamp is pulse light emission rather than continuous light, the following problems arise when it is used as the light source of image readout devices.
Specifically, if the processing cycle of an image input means such as a CCD is not synchronized with the inverter oscillation of a power supply device that supplies power to a dielectric barrier discharge fluorescent lamp, the light emission pulse number participating in image readout would not be constant per processing cycle of an image input means such as a CCD, and the brightness of the image that is read per processing cycle of an image input means such as a CCD would change. The mode of this change would be periodic based on the beat of the processing cycle of an image input means such as a CCD and of inverter oscillation. Accordingly, the image that is read would contain strip-like unevenness.
When a power supply device using a flyback inverter circuit is used, synchronization of the two is easily completed by initializing oscillation of an inverter oscillator via an external synchronization signal that exhibits a specific phase (for example, initial timing of the readout cycle) in the processing cycle of an image input means such as a CCD. However, various problems arise following the initialization of an oscillator by an external synchronization signal and synchronization of the two in full-bridge, half-bridge and push-pull inverter circuits.
The lighting circuit shown in FIG. 10 is explained here. The oscillation phase of inverter oscillators and the synchronization signal Sync fluctuate is based upon various factors. If the two do not overlap, the amount of light would fluctuate based solely on the fluctuation of the light emission cycle, but if the oscillation phases of the oscillator overlap the external synchronization signal due to fluctuation of the oscillation phase, the amount of light from the lamp would change. For example, an external synchronization signal is input, as shown by the broken line in FIG. 10, when synchronizing the processing cycle of an image input means such as a CCD with lamp light emission in the lighting circuit, and oscillation of a sawtooth wave oscillator must be initialized by an external synchronization signal.
As shown in FIG. 12(a), sawtooth wave oscillator Ocs is initialized by synchronization signal Sync and oscillation commences after a prescribed period of time when the external synchronization signal Sync is input at timing (following output of a pulse signal that inverts flipflop FF from comparator Cmp). Accordingly, and as shown in FIG. 12(c), flipflop FF is inverted, and as a result, a gate signal as shown in FIGS. 12(d) and 12(e) is input to the gate terminal of switch devices Q1, Q2 of an inverter circuit and the lamp 1 is lit. In contrast, and as shown in FIG. 13(a), sawtooth wave oscillator Ocs is initialized before the sawtooth wave reaches the fixed level of voltage Vs when external synchronization signal Sync is input at the timing, and pulse signal A that inverts the flipflop FF is lost. Thus, the output of flipflop FF becomes the output, as shown in FIG. 13(c). The timing at which switch devices Q1, Q2 turn on adopts the shape shown in FIGS. 13(d) and 13(e). Consequently, the optical power of the lamp decreases.
Usually, the frequency of external synchronization signal Sync and the oscillation frequency of the sawtooth wave oscillator are adjusted since the pulse oscillation phase/frequency fluctuate due to various factors. Even if the sawtooth wave oscillator is synchronized to a certain extent by external synchronization signal Sync, the oscillation phase of the oscillator will occasionally overlap external synchronization signal Sync. The light emission of the lamp would fluctuate if the oscillation phase of the oscillator and external synchronization signal Sync alternate between overlapping and not overlapping due to fluctuation of the oscillation phase of the oscillator and of external synchronization signal Sync.
Furthermore, the ON duration of switch devices Q1 (or Q2) would contract, as shown in FIGS. 14(a)-(d), when external synchronization signal Sync and the oscillation phase of the sawtooth wave oscillator overlap, and a large surge voltage would be applied to switch devices Q1 (or Q2). This is because the dielectric barrier discharge fluorescent lamp operates as a capacitor in terms of electrical circuitry, and a large current flows only for a period when the voltage applied to the lamp changes or for a period immediately after the change. Accordingly, a large current flows immediately after switch devices Q1, Q2 turn ON, but current that increases slowly dependent on the size of the inductance on the primary side of boosting transformer 2, so-called excitation current only, flows thereafter. This current is very small compared to the pulse current that flows immediately after aforementioned switch devices turn ON.
Specifically, the lamp current would become virtually zero when switch devices Q1, Q2 turn OFF if the switch devices Q1, Q2 are ON for a duration of time exceeding that required to change the lamp that operates as a capacitor. However, when current flows through the lamp, it would be obstructed by switch devices Q1, Q2 if the ON duration of switch devices Q1, Q2 is contracted, as shown in FIG. 14. For this reason, a large surge voltage would be applied to switch devices Q1, Q2. When a circuit that controls feedback of the input voltage +V through inverter circuit 3 as well as the current flowing through lamp 1 is mounted to stabilize the optical power of the lamp, the pulse signal that inverts the flipflop FF is repeatedly lost and not lost, which brings about instability in the feedback control circuit, thereby creating great optical fluctuation. These problems arise in half-bridge and full-bridge inverter circuits as well as push-pull inverter circuits. As indicated above, problems such as fluctuation in the optical power and surge in switch devices arise when an oscillator is initialized by an external synchronization signal in push-pull, half-bridge and full-bridge inverter circuits.
Accordingly, an object of the present invention is to overcome the disadvantages of the prior art in providing a light source device that is ideal for use as the light source of image readout devices in which a dielectric barrier discharge fluorescent lamp can emit light synchronized with an external synchronization signal without attendant fluctuation in the optical power. The light source device includes a dielectric barrier discharge fluorescent lamp that emits light utilizing ultraviolet light generated by dielectric barrier discharge and an inverter oscillator of a power supply device that supplies power to said dielectric barrier discharge fluorescent lamp functions as a variable frequency oscillator. The variable frequency oscillator is controlled so that oscillation signals from said variable frequency oscillator are phase locked with an external synchronization signal.
The light source device further includes an internal phase signal in which the periodic amplitude is roughly constant is generated at a specific phase of oscillation of a variable frequency oscillator of an inverter oscillator, and oscillation of the oscillator is initialized by an external synchronization signal. The voltage of the variable frequency oscillator is controlled as a function of the length of the periods when the effective duration of the external synchronization signal and the effective duration of the internal phase signal overlap and do not overlap. Oscillation of the variable frequency oscillator is phase-locked by controlling the oscillation frequency of the frequency oscillator.
The oscillator of the inverter of the power supply device that supplies power to a dielectric barrier discharge fluorescent lamp in the present invention functions as a variable frequency oscillator, as mentioned above, and the variable frequency oscillator is controlled so that the oscillation signal from said variable frequency oscillator would be phase locked with an external synchronization signal. Accordingly, the processing cycle of an image input means such as a CCD can be synchronized with oscillation of an inverter circuit and the light emission pulse number that participates in image readout can be held constant per processing cycle of an image input means such as a CCD. In addition, fluctuation of the optical power due to overlapping of the oscillation phases of an inverter oscillator and an external synchronization signal can be prevented.
Furthermore, an internal phase signal in which the periodic amplitude is roughly constant is generated at a specific phase of oscillation of a variable frequency oscillator of an inverter, oscillation of the oscillator is initialized by an external synchronization signal, the voltage of the variable frequency oscillator is controlled as a function of the length of the periods when the effective duration of the external synchronization signal and the effective duration of aforementioned internal phase signal overlap and do not overlap, and the oscillation frequency of the frequency oscillator is controlled, thereby permitting phase locking of the oscillation phase of the inverter to the external synchronization signal without use of a frequency divider. Consequently, the device can be completed inexpensively. Furthermore, the frequency range of the external synchronization signal is not restricted as a function of the variable range of the oscillation frequency of the variable frequency oscillator.